The ability of certain gases to absorb infrared radiation has been successfully utilized in developing instruments for gas sensing. In a simplified description, an infrared gas sensor comprises a narrow band infrared source (emitting radiation at the absorbing wavelength) and an infrared detector that are separated by a gas cell. The absorption is calculated from the infrared signals measured under zero gas (gas that does not have infrared absorption, e.g., nitrogen) I(0), and under the gas of interest I(G) using the relation, ##EQU1## However, this basic sensor configuration does not compensate for changes and deterioration of optical components with time and temperature. In practice, a reference channel is added to the sensor to correct for these potential problems. A reference channel is another band of infrared radiation that is not absorbed by the gas of interest. An ideal reference channel would use exactly the same optical path as the sensing channel and would not have any absorption by the gas of interest or by any other possible interfering gases. However, conventional sensors, in order be cost effective, do not satisfy the "ideal reference" conditions. For example, a sensor with one infrared source and two infrared detectors uses two different optical paths for the two channels that could change relative to one another over time and the two different detectors could age differently with time. These detectors could also have different temperature characteristics and need to be matched for optimum performance over temperature. The narrow band optical filters that are used to generate sensing and reference radiation are also sensitive to temperature. Therefore, the reference channel is not capable of correcting for any temperature drift caused by the pair of filters. In this example there are a total of four elements (2 detectors and 2 filters) that are sensitive to temperature and will contribute to temperature instability of the sensor. Another problem related to conventional gas sensing devices relates to the mechanical support structure needed to mount the infrared radiation source, detector, optical system and electronics in a reliable, durable and economical manner. In particular, the support structure for the optical components needs to be robust and mechanically stable. That is, it is important that there be no movement among the source, the optical assembly and the detector. Due to the relatively complex mechanical support required in conventional sensors for the radiation source and/or detector along with the light tube, such sensors are less robust than desired. For example, in a conventional light tube assembly which has a radiation source and a detector positioned at opposite ends of a tube, either one or both of the radiation source and the detector must be mounted remotely from the circuit board which contains the associated electronics and consequently additional wiring to the circuit board as well as mechanical supports are required. Further, such optical assemblies are not conducive for use with gases having different degrees of absorption. That is, the optical path between the IR light source and the detector is critical, particularly when gas concentrations with high IR absorption levels are of interest. Such gases require a sufficiently short optical path to provide suitable concentration resolution. On the other hand, when gas concentrations having low IR absorption levels are of interest, a relatively long optical path is required to provide suitable concentration resolution. It is difficult to adapt a tubular optical assembly for use with gases having different absorption characteristics.